1. Field of the Invention
This invention relates to a device for receiving light signals. More specifically, the invention relates to light receiving modules for optical communications which have application in high speed optical transmission systems.
2. Description of Related Art
As signal transmission speeds increase, a light element for converting optical signals into electrical signals in an optical communication system, for example, must be able to receive optical signals of large bandwidth and operate with high precision. At high transmission speeds, such as several gigabits per second, it is important to reduce parasitic capacitances and parasitic inductances to the utmost in connections between electronic elements, such as semiconductor elements, resistors and capacitors, and the light receiving element. If this is accomplished, it is possible to prevent bandwidth deterioration due to the wiring of the elements. Also, typically, deterioration of the light receiving element arises due to adverse reactions with oxygen in the air, etc. Therefore, it is necessary to hermetically seal the light receiving element to protect it from the external atmosphere. FIGS. 8(a) and 8(b) show a conventional light receiving module. FIG. 8(a) is a plan view of the module and FIG. 8(b) is a cross-sectional view taken along line A--A in FIG. 8(a). The module includes a light receiving element 802, electronic elements 804, 806 and 808 and a fiber holder 81. Light receiving element 802 is an optical semiconductor element, such as photodiode, which performs the photoelectric conversion. Electronic elements 804, 806 and 808 constitute a receiving circuit, provided with electrical signals from light receiving element 802. Elements 802, 804, 806 and 808 are disposed on a printed circuit board 82 which is incorporated in a case 83. Electrical conductive traces (not shown) are formed on board 82 to interconnect elements 802, 804, 806 and 808. Terminals 84, permitting connection to an external circuit (not shown), are arranged on a base of case 83. A side of case 83 has a hole 85 for fiber holder 81 to introduce an optical signal to light receiving element 802.
Fiber holder 81 includes an optical fiber 811 and a tube 812. Optical fiber 811 comprises an optical fiber strand 811a and a jacket (covering layer) 811b which covers strand 811a. Tube 812 is inserted into hole 85 of case 83. Optical fiber 811 is fixed to tube 812 at an optimum position relative to light receiving element 802 by solder 813 and adhesive 814.
Light receiving element 802 is mounted on board 82 with electronic elements 804, 806 and 808 in a densely packed arrangement. By densely packing the elements in case 83, it is possible to reduce electrical parasitic capacitances and parasitic inductances. Also optical fiber 811 is fixed so that an end of fiber strand 811a can be as close as possible to light receiving element 802. Therefore, it is possible to prevent the optical coupling efficiency from deteriorating due to any expansion of the output light wave from optical fiber 811.
In this conventional light receiving module, case 83 is made airtight by a cover 86 to prevent characteristics of light receiving element 802 from deteriorating. However, it is difficult to make fiber holder 81 completely airtight because of poor solder connections between tube 812 and optical fiber 811. Voids remain in solder 813. This produces a yield decrease during production of optical holder 81. To improve the yield, flux may be used to decrease surface tension and remove an oxide film on the solder. However, flux vapor, released during heating, deteriorates the optical coupling due to contamination of light receiving element 802 and an end surface of fiber strand 811a. Also, since fiber strand 811a extends beyond tube 812 as shown in FIG. 8(a) and 8(b), the end of fiber strand 811a is easy to move and strand 811a easily sags as a result of external vibrations and impact.
Thus, the optical coupling efficiency varies. Also, the airtight seal is not always reliable, particularly when subjected to changes in temperature. Differences in the thermal expansion coefficient between the glass fiber and solder weaken the bonding. In addition, when solder is heated to fix optical fiber 811, jacket 811b is easy to damage. This decreases the production yield of the light receiving module.
Moreover, when fiber strand 811a is inserted in tube 82 and fiber strand 811a is brought close to light receiving element 802 to secure optical fiber 811, if fiber strand 811a hits light receiving element 802, the fiber may break and the light receiving element 802 may be damaged. The effective diameter of light receiving element 802 for high-speed communication is less than or equal to 100 82 m, it is necessary to bring fiber strand 811a within a distance less than or equal to several hundred .mu.m from light receiving element 802. Thus, fiber strand 811a often bumps light receiving element 802 during alignment.